Investigations on the Propagation of Siren Sound Signals in Town and Country Areas

Sound level measurements have been carried out on electrical 5-kW-sirens for a long period in areas of a large town covered with buildings located in diverse ways. Further measurements have been performed on compressed air sirens in a rural district, and by a sampling manner in three other large towns. It was the aim of this investigation to examine the influence, of the kind of shielding given by buildings, and of the wind, on the propagation of siren signals.

In streets with direct sound emission from the sirens the level decreased about 8 dB on the average for doubling the distance. This holds for distances up to 250 m. In the range from 250 m to 500 m the decrease amounted to about 15 dB. Signal observations behind buildings with several floors have shown that besides the 1/r decrease of the signal level there must be added a quantity which depends on the distance of the siren and on the kind of shielding given by buildings. In very unfavourable cases this quantity amounted to about 11 dB at a distance of 125 m and to 23 dB at 250 m. The measured results could be affirmed by model experiments in which the shadowing effect of single buildings has been simulated.

Among the parameters of the wind which could influence the propagation of sound only the direction of the wind has been considered. A statistically definite influence could be proved only for measuring distances greater than 500 m from the sirens.

The measurements on compressed air sirens have shown a l/r decrease of the sound level combined with a supplementary attenuation of about 17 dB up to distances of 4 km.